Instinct (本能, Honō), sometimes just called Homunculus as an insult, is Ten Tailed Fox's primary antagonist to Ryan Blackwood and his cousin, Matthew Fenn, both of whom are his primary protagonists. He is a Homunculus that was created as a test subject for the Arcadia Armed Forces, which was looking in to the possible use of Homunculus as soldiers instead of alchemists, letting the alchemists do their job as scientists. This failed, as Instinct had no intention of being controlled.

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Instinct has long straight, white hair, reaching down to his waist, and a large hole, decorated with markings stretching from the hole, across his chest and shoulders. He also has retractable claws on his fingertips and toes, small furry purple tufts, two starting from his collarbone and stretching around his neck, and one each on his wrists and ankles, as well as seemingly white skin. At almost all times, he wears a mask and his face is shaped the same, it covers Instinct's entire head and it also has crimson red markings on it; it has four spike-shaped marks stretched, two of them stopping by the forehead, and the other two stretching past the eyes and teeth, going past his chin and joining with the marks on his chest. There are also small marks resembling a heart, but with the top and bottom open, extending from the inner corners of his eyes. The mask features two long forward-pointing horns with red strips extending from their center towards the tips.

His overall appearance, without the mask, is practically the same as Ryan's; they look very similar, only with his eyes differing in color and his hair differing in both color and length. He is a fairly tall and lean-built being with pure white skin and hair. Black pupils, grey irises and black sclera. He also has black nails. He wears a long-sleeved, ankle-length white coat (or shikakusho) with black lining that is closed at his chest, then left to flare out into ragged ends.

Instinct is and has always been very secretive, keeping his thoughts and opinions mostly to himself. He is also a very cunning and intelligent homunculus. This is attributed to how long his life has actually been, being older than all of the current military leaders and politicians; this is because he was born the very year that alchemy was brought to Arcadia by Alphonse Elric in 1926. His cunning is clearly shown in his orchestrating of the creation of a Philosopher's Stone and his plans to create a nationwide Stone that could effect alchemists in all of Arcadia. Furthermore, his cunning is shown in not only orchestrating these events, but remaining unsuspected for so long.

He is a visionary, believing unlike the homunculus that came before him in that he doesn't wish to become a human nor does he desire all of the "knowledge and power of God", but rather he desires domination. He wishes to become the greatest non-human alchemist on Earth and use that power to put every human nation below his rule. Being the result of a Human Transmutation, Instinct has knowledge of the Truth but does not care for it, claiming that "Its Truth is not his truth" and therefore passes it off as nonsense. As possibly a smarter being that Father, Instinct has discovered a new form of alchemy called Energy Transmutation, a rare power that grants the user the right to defy the Law of Conservation of Mass should that user have passed through the Gate and seen the Truth as well as possessing a Philosopher's Stone. He discovered this after the creation of his own Philosopher's Stone. True to his name, Instinct believes the sole most important thing to a human or homunculus is their instinct: the inherent inclination of a living organism toward a particular behavior. The fixed action patterns are unlearned and inherited. He often claims that his actions were based on his instinct, and he judges any human that resists their instinct to be primal and garbage.

Instinct is capable of destroying a large portion of the ground beneath his feet with the wake of a single slash of his sword. He is capable of easily tearing off a human soldier's arm without resistance from the soldier's armor or equipment. He is capable of crushing basic alchemical formulas and attacks and force them to collapse before his foe even gets a chance to execute it. He easily slashed a rock golem, created by alchemy, from his left shoulder to his lower right hip through his rock armor. He is also able to effortlessly throw a large portion of rock with a single hand. Like most Homunculus, Instinct has very high regenerative power, longevity, and, as described above, various superhuman feats.

One of Instinct's most astounding ability is his growth rate: he grasps the basic control of advanced techniques in days where it would take several years for even the most gifted alchemists. Shortly after being "born" via Human Transmutation and with no previous training and instruction, he slaughtered dozens of the alchemists responsible for his creation to forge his own Philosopher's Stone, and on top of this, he learned how to use it within several days after its creation. Another example of his growth rate is the amount of time he took to learn alchemy and master it, which he learns in little less than a day, though this may be because, like all other homunculus, he came from the Gate itself and therefore had innate knowledge of alchemy and its applications. He also gains considerable experience in the use of swords in combat while training alone in the mountains of Arcadia after his killing of his creators, which was done over the same period as his learning of alchemy. Besides his improvements in his alchemy prowess, Instinct was also shown able to greatly control his own unique Homunculus ability, which all homunculus receive at their "birth". His power is "alchemy nullification", which basically means that he can emit a field around him that can nullify all alchemy but his own, leaving his foes to basic combat training. His power, however, does not restrict use of Alkahestry or weaker methods of alchemy, like Scar's destruction alchemy.

Physically, Instinct is in top form even when he is not using alchemy or his Philosopher's Stone. Instinct has been trained in martial arts, though this is mostly his own training regiments. He has repeatedly shown great agility and coordination in his attacks. In addition to having powerful kicks and punches, Instinct has also shown proficiency in counterattacks, takedowns, and body locks. He has repeatedly been able to catch an opponent's weapon strikes with a single hand. He also regularly defeats multiple soldiers and lower-level alchemists, and is a better martial artist than Ryan Blackwood, who is ranked one of the strongest alchemists in all of Arcadia.

Instinct using alchemy to form a black sword from the earth behind him.

Instinct is one of the few homunculus who can preform alchemy. This is because, when he was created, the corpse of a prison inmate was used as the focus of the Human Transmutation that created him. Because he was once part of the Gate, he has vast knowledge of alchemy and can use it without a Transmutation Circle and, due to his Philosopher's Stone, he can ignore the Law of Equivalent Exchange to preform alchemy. He does not use alchemy too often. The most common form he uses is to transmute the earth beneath his feet into a solid black sword to use in sword combat. Just like his predecessor, Father, he can release blasts of alchemical energy in the form of nuclear fusion between the horns of his mask. Like most alchemists, he will use alchemy to manipulate his surroundings.

As the inventor of Energy Transmutation, he is its most proficient user, using its power to argument the energy behind his physical attacks and to release bursts of energy in the form of blasts. Though he can complete Human Transmutation via his Stone, he choses not to because it is unconventional for him.

In terms of fighting style while versing low level alchemists or non-alchemy using humans, Instinct relies mainly on swordplay, and thus would be considered a master of swordsmanship. Aside from his initial training, he is basically on a "learn as you go" training regimen thus his skill has developed by fighting rather than formal instruction, becoming even more capable with every fight as he did with his alchemy skills.

Instinct was created, in 1926, as the result of an experiment by the Arcadian military to create alchemy-using Homunculus as soldiers for war apart from alchemists, allowing normal alchemists to do their jobs as scientists. To do this, they brought in an inmate, slated for execution an used him in the Human Transmutation to create the homunculus that would become Instinct. This failed, however, because the Homunculus turned on his creators, using their own guns to slaughter them and then forge a Philosopher's Stone out of their souls.